The invention relates to an input light-coupling device using a dielectric layer containing a pattern of dielectric contrast distributed in at least two dimensions. Hereinafter the term xe2x80x9cdielectric constantxe2x80x9d is assumed to refer to the real part of the dielectric constant.
A fundamental limitation to the speed of telecommunications is the need to convert an optical signal to an electrical signal. One way to avoid this limitation is to process the signal optically. Optical signal processing requires the transfer of an optical signal from either an optical fiber or free-space (depending on the application) onto a chip. Typically, the input coupling of light introduces losses to the optical signal.
Optical signals propagating through a fiber are usually inserted onto a semiconductor chip by end-firing the optical signal into the waveguide, a practice that can lead to significant insertion losses. For instance, optical fibers are xe2x80x9cbutt-coupledxe2x80x9d to the end of the waveguide using an epoxy. An optical signal carried though free space needs to be focused and inserted at the edge of a semiconductor waveguide. The insertion losses are due to the dielectric contrast and reflectivity associated with the interface between the fiber, or free space, and the semiconductor.
Gratings may be defined in the high-dielectric semiconductor to improve the coupling efficiency, as is described in further detail in Loewen and Popov., Diffraction Gratings and Applications, (Marcel Dekker, New York, 1997). An example of a grating coupler is provided in Backlund et al., xe2x80x9cMultifunctional Grating Couplers for Bidirectional Incoupling into Planar Waveguides.xe2x80x9d, IEEE Phot. Tech. Lett. 12, 314 (2000). The grating consists of a one-dimensional periodic perturbation to the dielectric constant. The optical signal no longer needs to be launched at the end of the waveguide but rather may couple from above the grating. Efficient input coupling of light into such structures, however, is limited by the single direction of the periodicity.
Accordingly, the invention provides a new design for the input coupling of light from a low dielectric medium (such as an optical fiber or free space) to a higher dielectric material (such as a semiconductor). The input coupling structure includes a pattern of dielectric contrast distributed in at least two dimensions, the exact design of which may be scaled, within fabrication limits, to apply to any wavelength of input light. The pattern may be fabricated in a dielectric layer that contains other optical components, e.g., a semiconductor chip, such as waveguides. Efficient input coupling may occur in as many directions as dictated by the symmetry of the structure. Multiple input directions allow an increased flexibility in integrated optic design, an increase in the optical component packing density, and a possible increase in the overall coupling efficiency.
In accordance with an exemplary embodiment of the invention, there is provided a device comprising a dielectric layer containing a two-dimensional periodicity in the dielectric constant of the layer, and separated from a substrate by a low-dielectric spacer layer and a mirror consisting of alternating layers of low and high-dielectric material. Input coupling into directions of high-symmetry is experimentally observed.